The present invention relates to semiconductor structures, and more particularly to semiconductor structures having particles on a semiconductor substrate surface.
A class of solid state devices commonly used in manufacturing integrated circuits is known as the MOS or MIS transistor. The name derives from their basic structure which employs a semiconductor substrate such as silicon or gallium arsenide called "S", a conductive gate electrode usually made of polycrystalline (poly) silicon or metal called "M", and in between the two an insulating layer called "I" which is often comprised of silicon dioxide called "O". Large chips such as multi-megabit memories utilize millions of MOS transistors, all of which must work reliably within specification limits.
It is important to note that the insulating layer in an MOS transistor is very thin, often less than 150 .ANG. thick, and it is generally formed by heating the silicon substrate in an oxidizing atmosphere. In the liquid chemicals used for etching, cleaning and washing, and in the equipment and the supply gases used for oxidation there are particulates. Some of these particulates will remain on the surface of a silicon wafer when it is ready to begin its high temperature oxidation cycle. Particulates smaller than 3500 .ANG. are not detectable by production-compatible techniques. These small particulates are still very large compared to the oxide thickness to be grown. Accordingly, a small particle on the silicon surface during a thermal oxidation cycle can cause a large localized perturbation in oxide properties.
For example, refractory particulates may simply tend to mask the oxidation process; if a particulate is subsequently removed, a thin spot in the oxide remains. Metallic particulates will tend to react with the silicon and oxygen, producing a region that is not the desired SiO.sub.2 insulator, but rather a metal-silicon-oxygen composite. If a defective region in the insulator of a device is large enough, its MOS I-V characteristic will exhibit higher leakage current than the rest of the device; this is measurable. However, in most cases the dielectric defect is so small that even though the current density flowing through it is very high compared to the rest of the MOS structure, its current is less than that carried by the remaining undefected structure. For example, a 100 .ANG. defect carrying 10.sup.4 times as much current density would be unnoticed in a 20 .mu.m.sup.2 MOS device.
Nevertheless, SiO.sub.2 exhibits a well-known wear-out phenomenon; it breaks down electrically as a result of electric current flowing through it. Breakdown depends upon how much electronic charge density, e.g. coulombs per cm.sup.3, has passed through the insulator. For steady state conditions, this equals current density times time. This is called the charge-to-breakdown, or Q.sub.BD, effect. Accordingly, an MOS chip may pass final test, but fail burn in or exhibit field failure as a result of a small particle-induced, electrically-invisible defect in the gate oxide of one of its thin dielectric MOS regions.
Consequently, what is needed is a method for minimizing the undesirable impact and consequences of small particles in semiconductor processes.